Shale is a fine-grained sedimentary rock formed from mud containing little/no organic matter, clay minerals and fine fragments of other minerals. It is classified in accordance with total organic carbon (TOC). The TOC in shale represent the total carbon remained after expelling Inorganic carbon in a sample and indicates the sediments potential for hydrocarbon generation. TOC% enables the classification of shales into four categories:

Shale containing less than 0.5% TOC is considered as poor source rock.
Shale containing a TOC between 0.5% and 1% indicate fair source rock.
Shale containing a TOC value between 1% and 2% indicate good source rock.
Shale containing a TOC value above 2% often indicate a highly reducing environment with excellent source potential.